2007-02-04

So you by now know, I hope, that the Intergovernmental Panel on Climate Change has issued it's Fourth Assessment Report of working group I (i.e. the "physical scientists). Okay, actually, it has only released an abstraction of the AR4 called the Summary for Policymakers. The full report will be available in a couple months. For now, get the summary from the UCAR site [PDF].

There are a lot of issues about this report and the IPCC in general that I'm tempted to start spouting. Instead, I'm going to let those thoughts roll around a little more, and perhaps wait for the full report. For today, I just want to point out some key points, about the summary, along with some cautionary words.

The report is written by climate scientists (so is the summary). All the writers, both lead authors and contributing authors, work on a voluntary basis. I think the idea, at least for the north American and European scientists, is that the IPCC is an important way for scientists to interface with the policymakers and general public, and that working on the report is an important outreach activity. The IPCC as an entity is organized under the auspices of the World Meteorological Organization (WMO) and the United Nations Nations Environment Programme (UNEP), and as such it is open to members of those organizations. That means that a lot of scientists are eligible to contribute to the report, but it also means that there are a lot of governments that have vested interests in what the report ultimately says.

Do governments influence the content of the report? Well, from what I've heard ('round the proverbial water cooler), there is very little direct interaction with world governments. Drafts of the reports are sent out to tons and tons of people, including governments, so there are notes sent back. Those notes have to be addressed individually, but I'm sure that most of them are insignificant and are basically ignored. The indirect influence is probably more important. The scientists writing the report are aware of the political/societal implications, and try to protect themselves by explicitly avoiding making prescriptive suggestions; they don't say what to do about climate change, they just evaluate the scientific evidence to evaluate the extent of climate change and projections of future change. The indirect influence of governments and economics makes the authors, in my opinion, even more conservative in their language than scientists normally are. I think the report generally does not embrace more extreme projections and predictions, which is to their credit, but is a caveat when reading the report or the summary. That is, some of the key points are probably more conservative than individual scientists would suggest.

Of course, there is also pressure to put new and important results into the report. This pressure comes less from external sources and more from the drive to show how much we've learned since the last report. There are a few points in the summary that I was surprised to see, not because they aren't important, but because I think there have to be many, many caveats. The two examples that stick out are (1) that tropical cyclones are getting more intense with global warming and (2) that patterns of precipitation will change in the future, with specific patterns emerging as robust. I'll blog more about both of these in the future, but here I'll just say that I look forward to reading the specifics in the full report. I generally believe the first claim, while the second one seems extremely poorly constrained by climate models.

Even with these opposing pressures, the results of the summary are largely unsurprising and in line with the Third Assessment Report (2001). The lower range of climate sensitivity has inched up a bit from 1.5C to 2.0 C. That basically has had to move up as we've seen more and more warming over the past decade. They are also making the upper range of possible sensitivity more hazy by mentioning some projections of greater than 5C or so, even though they don't necessarily incorporate those very sensitive projections in the non-analysis that goes into writing the report. (I'll also talk about what I mean by non-analysis in a future post.)

So, yes, global warming is happening. Oh, and yes, it is because of humans emitting carbon dioxide into the atmosphere.

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